This invention relates generally to magnetic resonance imaging of an object having different chemical species therein, such as fat and water, and more particularly the invention relates to species imaging in the presence of magnetic field heterogeneity.
Reliable and uniform fat suppression is essential for accurate diagnoses in many areas of MRI. This is particularly true for sequences such as fast spin-echo (FSE), steady-state free precession (SSFP) and gradient echo (GRE) imaging, in which fat is bright and may obscure underline pathology. Although conventional fat saturation may be adequate for areas of the body with relative homogeneous Bo field, there may be many applications in which fat saturation routinely fails. This is particularly true for extremity imaging, off-isocenter imaging, large field of view (FOV) imaging, and challenging areas such as the brachial plexus and skull based, as well as many others. Short-TI inversion recovery (STIR) imaging provides uniform fat suppression, but at a cost of reduced signal-to-noise ratio (SNR) and mixed contrast that is dependent on T1. This latter disadvantage limits STIR imaging to T2 weighted (T2W) applications, such that current T1 weighted (T1 W) applications rely solely on conventional fat-saturation methods. Another fat suppression technique is the use of spectral-spatial or water selective pulses; however, this method is also sensitive to field inhomogeneities.
“In and Out of Phase” Imaging was first described by Dixon in “Simple Proton Spectroscopic Imaging”, Radiology (1984) 153:189-194, and was used to exploit the difference in chemical shifts between water and fat and in order to separate water and fat into separate images. Glover et al. further refined this approach, described in Glover G., “Multipoint Dixon Technique for Water and Fat Proton and Susceptibility Imaging”, Journal of Magnetic Resonance Imaging (1991) 1:521-530, with a 3-point method that accounts for magnetic field inhomogeneities created by susceptibility differences. This method was applied with FSE imaging by acquiring three images with the readout centered at the spin-echo for one image and symmetrically before and after the spin-echo in the subsequent two images.
Dynamic MRI repeatedly acquires images at the same locations. A common use of dynamic imaging is the investigation of the time course of tissue contrast after injection of a paramagnetic contrast agent. For example, contrast-enhanced liver MRI is a typical dynamic imaging application, where the tumor appears brighter than the normal tissues after the use of contrast. Other more sophisticated applications, such as breast imaging, may require the examination of the signal changes during a period of time to determine the malignancy of the lesions. Repeated acquisitions must be fast enough to provide high temporal resolutions of the signal changes. In addition, fat signal should be eliminated for better lesion conspicuity. Therefore a reliable fat suppression technique is needed for dynamic imaging, which at the same time should add minimum scan time cost to maintain a high temporal resolution.